Network Working Group C. Huitema
Internet-Draft Private Octopus Inc.
Intended status: Informational June 29, 2018
Expires: December 31, 2018
DNS-SD Privacy Scaling Tradeoffsdraft-huitema-dnssd-privacyscaling-01
Abstract
DNS-SD (DNS Service Discovery) normally discloses information about
both the devices offering services and the devices requesting
services. This information includes host names, network parameters,
and possibly a further description of the corresponding service
instance. Especially when mobile devices engage in DNS Service
Discovery over Multicast DNS at a public hotspot, a serious privacy
problem arises.
The draft currently progressing in the DNS-SD Working Group assumes
peer-to-peer pairing between the service to be discovered and each of
its clients. This has good security properties, but creates scaling
issues, because each server needs to publish as many announcements as
it has paired clients. This leads to large number of operations when
servers are paired with many clients.
Different designs are possible. For example, if there was only one
server "discovery key" known by each authorized client, each server
would only have to announce a single record, and clients would only
have to process one response for each server that is present on the
network. Yet, these designs will present different privacy profiles,
and pose different management challenges. This draft analyses the
tradeoffs between privacy and scaling in a set of different designs,
using either shared secrets or public keys.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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requesting identities along with information about the offered and
requested services. Parts of the published information can seriously
breach the users' privacy. These privacy issues and potential
solutions are discussed in [KW14a] and [KW14b].
A recent draft [I-D.ietf-dnssd-privacy] proposes to solve this
problem by relying on device pairing. Only clients that have paired
with a device would be able to discover that device, and the
discovery would not be observable by third parties. This design has
a number of good privacy and security properties, but it has a cost,
because each server must provide separate annoucements for each
client. In this draft, we compare scaling and privacy properties of
three different designs:
o The individual pairing defined in [I-D.ietf-dnssd-privacy],
o A single server discovery secret, shared by all authorized
clients,
o A single server discovery public key, known by all authorized
clients.
After presenting briefly these three solutions, the draft presents
the scaling and privacy properties of each of them.
2. Privacy and Secrets
Private discovery tries to ensure that clients and servers can
discover each other in a potentially hostile network context, while
maintaining privacy. Unauthorized third parties must not be able to
discover that a specific server or device is currently present on the
network, and they must not be able to discover that a particular
client is trying to discover a particular service. This cannot be
achieved without some kind of shared secret between client and
servers. We review here three particular designs for sharing these
secrets.
2.1. Pairing secrets
The solution proposed in [I-D.ietf-dnssd-privacy] relies on pairing
secrets. Each client obtains a pairing secret from each server that
they are authorized to use. The servers publish announcements of the
form "nonce|proof", in which the proof is the hash of the nonce and
the pairing secret. The proof is of course different for each
client, because the secrets are different. For better scaling, the
nonce is common to all clients, and defined as a coarse function of
time, such as the current 30 minutes interval.
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Clients discover the required server by issuing queries containing
the current nonce and proof. Servers respond to these queries if the
nonce matches the current time interval, and if the proof matches the
hash of the nonce with one of the pairing key of an authorized
client.
2.2. Group public keys
In contrast to pair-wise shared secrets, applications may associate
public and private key pairs with groups of equally authorized
clients. This is identical to the pairwise sharing case if each
client is given a unique key pair. However, this option permits
multiple users to belong to the same group associated with a public
key, depending on the type of public key and cryptographic scheme
used. For example, broadcast encryption is a scheme where many
users, each with their own private key, can access content encrypted
under a single broadcast key. The scaling properties of this variant
depend not only on how private keys are managed, but also on the
associated cryptographic algorithm(s) by which those keys are used.
2.3. Shared symmetric secret
Instead of using a different secret for each client as in
Section 2.1, another design is to have a single secret per server,
shared by all authorized clients of that server. As in the previous
solution, the servers publish announcements of the form
"nonce|proof", but this time they only need to publish a single
announcement per server, because each server maintains a single
discovery secret. Again, the nonce can be common to all clients, and
defined as a coarse function of time.
Clients discover the required server by issuing queries containing
the current nonce and proof. Servers respond to these queries if the
nonce matches the current time interval, and if the proof matches the
hash of the nonce with one of the discovery secrets.
2.4. Shared public key
Instead of a discovery secret used in Section 2.3, clients could
obtain the public keys of the servers that they are authorized to
use.
Many public key systems assume that the public key of the server is,
well, not secret. But if adversaries know the public key of a
server, they can use that public key as a unique identifier to track
the server. Moreover, they could use variations of the padding
oracle to observe discovery protocol messages and attribute them to a
specific public key, thus breaking server privacy. For these
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reasons, we assume here that the discovery public key is kept secret,
only known to authorized clients.
As in the previous solution, the servers publish announcements of the
form "nonce|proof", but this time they only need to publish a single
announcement per server, because each server maintains a single
discovery secret. The proof is obtained by either hashing the nonce
with the public key, or using the public key to encrypt the nonce --
the point being that both clients and server can construct the proof.
Again, the nonce can be common to all clients, and defined as a
coarse function of time.
The advantage of public key based solutions is that the clients can
easily verify the identity of the server, for example if the service
is accessed over TLS. On the other hand, just using standard TLS
would disclose the certificate of the server to any client that
attempts a connection, not just to authorized clients. The server
should thus only accept connections from clients that demonstrate
knowledge of its public key.
3. Scaling properties of different solutions
To analyze scaling issues we will use the following variables:
N: The average number of authorized clients per server.
G: The average number of authorized groups per server.
M: The average number of servers per client.
P: The average total number of servers present during discovery.
The big difference between the three proposals is the number of
records that need to be published by a server when using DNS-SD in
server mode, or the number of broadcast messages that needs to be
announced per server in mDNS mode:
Pairing secrets: O(N): One record per client.
Group public keys: O(G): One record per group.
Shared symmetric secret: O(1): One record for all (shared) clients.
Shared public key: O(1): One record for all (shared) clients.
There are other elements of scaling, linked to the mapping of the
privacy discovery service to DNS-SD. DNS-SD identifies services by a
combination of a service type and an instance name. In classic
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mapping behavior, clients send a query for a service type, and will
receive responses from each server instance supporting that type:
Pairing secrets: O(P*N): There are O(P) servers present, and each
publishes O(N) instances.
Group public keys: O(P*G): There are O(P) servers present, and each
publishes O(G) instances.
Shared symmetric secret: O(P): One record per server present.
Shared public secret: O(P): One record per server present.
The DNS-SD Privacy draft suggests an optimization that considerably
reduces the considerations about scaling of responses -- see section4.6 of [I-D.ietf-dnssd-privacy]. In that case, clients compose the
list of instance names that they are looking for, and specifically
query for these instance names:
Pairing secrets: O(M): The client will compose O(M) queries to
discover all the servers that it is interested in. There will be
at most O(M) responses.
Group public keys: O(M): The client will compose O(M) queries to
discover all the servers that it is interested in. There will be
at most O(M) responses.
Shared symmetric secret: O(M): Same behavior as in the pairing
secret case.
Shared public secret: O(M): Same behavior as in the pairing secret
case.
Finally, another element of scaling is cacheability. Responses to
DNS queries can be cached by DNS resolvers, and mDNS responses can be
cached by mDNS resolvers. If several clients send the same queries,
and if previous responses could be cached, the client can be served
immediately. There are of course differences between the solutions:
Pairing secrets: No caching possible, since there are separate
server instances for separate clients.
Group public keys: Caching is possible for among members of a group.
Shared symmetric secret: Caching is possible, since there is just
one server instance.
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Shared public secret: Caching is possible, since there is just one
server instance.
4. Comparing privacy posture of different solutions
The analysis of scaling issues in Section 3 shows that the solutions
base on a common discovery secret or discovery public key scale much
better than the solutions based on pairing secret. All these
solutions protect against tracking of clients or servers by third
parties, as long as the secret on which they rely are kept secret.
There are however significant differences in privacy properties,
which become visible when one of the clients becomes compromised.
4.1. Effects of compromized client
If a client is compromised, an adversary will take possession of the
secrets owned by that client. The effects will be the following:
Pairing secrets: With a valid pairing key, the adversary can issue
queries and parse announcements. It will be able to track the
presence of all the servers to which the compromised client was
paired. It may be able to track other clients of these servers if
it can infer that multiple independent instances are tied to the
same server, for example by assessing the IP address associated
with a specific instance. It will not be able to impersonate the
servers for other clients.
Group public keys: With a valid group private key, the adversary can
issue queries and parse announcements. It will be able to track
the presence of all the servers with which the compromised group
was authenticated. It may be able to track other clients of these
servers if it can infer that multiple independent instances are
tied to the same server, for example by assessing the IP address
associated with a specific instance. It will not be able to
impersonate the servers for other clients or groups.
Shared symmetric secret: With a valid discovery secret, the
adversary can issue queries and parse announcements. It will be
able to track the presence of all the servers that the compromised
client could discover. It will also be able to detect the clients
that try to use one of these servers. This will not reveal the
identity of the client, but it can provide clues for network
analysis. The adversary will also be able to spoof the server's
announcements, which could be the first step in a server
impersonation attack.
Shared public secret: With a valid discovery public key, the
adversary can issue queries and parse announcements. It will be
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able to track the presence of all the servers that the compromised
client could discover. It will also be able to detect the clients
that try to use one of these servers. This will not reveal the
identity of the client, but it can provide clues for network
analysis. The adversary will not be able to spoof the server's
announcements, or to impersonate the server.
4.2. Revocation
Assume an administrator discovers that a client has been compromised.
As seen in Section 4.1, compromising a client entails a loss of
privacy for all the servers that the client was authorized to use,
and also to all other users of these servers. The worse situation
happens in the solutions based on "discovery secrets", but no
solution provides a great defense. The administrator will have to
remedy the problem, which means different actions based on the
different solutions:
Pairing secrets: The administrator will need to revoke the pairing
keys used by the compromised client. This implies contacting the
O(M) servers to which the client was paired.
Group public key: The administrator must revoke the private key
associated with the compromised group members and, depending on
the cryptographic scheme in use, generate new private keys for
each existing, non-compromised group member. The latter is
necessary for public key encryption schemes wherein group access
is permitted based on ownership (or not) to an included private
key. Some public key encryption schemes permit revocation without
rotating any non-compromised group member private keys.
Shared symmetric secret: The administrator will need to revoke the
discovery secrets used by the compromised client. This implies
contacting the O(M) servers that the client was authorized to
discover, and then the O(N) clients of each of these servers.
This will require a total of O(N*M) management operations.
Shared public secret: The administrator will need to revoke the
discovery public keys used by the compromised client. This
implies contacting the O(M) servers that the client was authorized
to discover, and then the O(N) clients of each of these servers.
Just as in the case of discovery secrets, this will require O(N*M)
management operations.
The revocation of public keys might benefit from some kind of
centralized revocation list, and thus may actually be easier to
organize than simple scaling considerations would dictate.
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Internet-Draft DNS-SD Privacy Scaling Tradeoffs June 20184.3. Effect of compromized server
If a server is compromised, an adversary will take possession of the
secrets owned by that server. The effects are pretty much the same
in all configurations. With a set of valid credentials, the
adversary can impersonate the server. It can track all of the
server's clients. There are no differences between the various
solutions.
As remedy, once the compromise is discovered, the administrator will
have to revoke the credentials of O(N) clients, or O(G) groups,
connected to that server. In all cases, this could be done by
notifying all potential clients to not trust this particular server
anymore.
5. Summary of tradeoffs
In the preceding sections, we have reviewed the scaling and privacy
properties of three possible secret sharing solutions for privacy
discovery. The comparison can be summed up as follow:
+-------------------------+---------+------------+-------------+
| Solution | Scaling | Resistance | Remediation |
+-------------------------+---------+------------+-------------+
| Pairing secret | Poor | Bad | Good |
| Group public key | Medium | Bad | Maybe |
| Shared symmetric secret | Good | Really bad | Poor |
| Shared public secret | Good | Bad | Maybe |
+-------------------------+---------+------------+-------------+
Table 1: Comparison of secret sharing solutions
All four types of solutions provide reasonable privacy when the
secrets are not compromised. They all have poor resistance to the
compromise of a client, as explained in Section 4.1, but sharing a
symmetric secret is much worse because it does not prevent server
impersonation. The pairing secret solution scales worse than the
discovery secret and discovery public key solutions. The group
public key scales as the number of groups for the total set of
clients; this depends on group assignment and will be intermediate
between the pairing secret and shared secret solutions. The pairing
secret solution can recover from a compromise with a smaller number
of updates, but the public key solutions may benefit from a simple
recovery solution using some form of "revocation list".
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[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[SIGMA] Krawczyk, H., "SIGMA: The 'SIGn-and-MAc'approach to
authenticated Diffie-Hellman and its use in the IKE
protocols", 2003, <http://link.springer.com/content/pdf/10.1007/978-3-540-45146-4_24.pdf>.
[Wu16] Wu, D., Taly, A., Shankar, A., and D. Boneh, "Privacy,
discovery, and authentication for the internet of things",
2016, <https://arxiv.org/pdf/1604.06959.pdf%22>.
Appendix A. Survey of Implementations
This section surveys several private service discovery designs in the
context of the threat model detailed above.
A.1. DNS-SD Privacy Extensions
Huitema and Kaiser [I-D.ietf-dnssd-privacy] decompose private service
discovery into two stages: (1) identify specific peers offering
private services, and (2) issue unicast DNS-SD queries to those hosts
after connecting over TLS using a previously agreed upon pre-shared
key (PSK), or pairing key. Any out-of-band pairing mechanism will
suffice for PSK establishment, though the authors specifically
mention [I-D.ietf-dnssd-pairing] as the pairing mechanism. Step (1)
is done by broadcasting "private instance names" to local peers,
using service-specific pairing keys. A private instance name N' for
some service with name N is composed of a unique nonce r and
commitment to r using N_k. Commitments are constructed by hashing
N_k with the nonce. Only owners of N_k may verify its correctness
and, upon doing so, answer as needed. The draft recommends
randomizing hostnames in SRV responses along with other identifiers,
such as MAC addresses, to minimize likability to specific hosts.
Note that this alone does not prevent fingerprinting and tracking
using that hostname. However, when done in conjunction with steps
(1) and (2) above, this mitigates fingerprinting and tracking since
different hostnames are used across venues and real discovered
services remain hidden behind private instance names.
After discovering its peers, a node will directly connect to each
device using TLS, authenticated with a PSK derived from each
associated pairing key, and issue DNS-SD queries per usual. DNS
messages are formulated as per [RFC7858].
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As an optimization, the authors recommend that each nonce be
deterministically derived based on time so that commitment proofs may
be precomputed asynchronously. This avoids O(N*M) computation, where
N is the number of nodes in a local network and M is the number of
per-node pairings.
This system has the following properties:
1. Symmetric work load: clients and servers can pre-compute private
instance names as a function of their pairing secret and
predictable nonce.
2. Mutual identity privacy: Both client and server identities are
hidden from active and passive attackers that do not subvert the
pairing process.
3. No client set size hiding: The number of private instance names
reveals the number of unique pairings a server has with its
clients. (Servers may pad the list of records with random
instance names, though this introduces more work for clients.)
4. Unlinkability: Private service names are unlinkable to post-
discovery TLS connections. (Note that if deterministic nonces
repeat, servers risk linkability across private service names.)
5. No fingerprinting: Assuming servers use fresh nonces per private
instance name, advertisements change regularly.
A.2. Private IoT
Boneh et al. [Wu16] developed an approach for private service
discovery that reduces to private mutual authentication. Moreover,
it should be infeasible for any adversary to forge advertisements or
impersonate anyone else on the network. Specifically, service
discoverers only wish to reveal their identity to services they
trust, and vice versa. Existing protocols such as TLS, IKE, and
SIGMA [SIGMA] require that one side reveal its identity first. Their
approach first allocates, via some policy manager, key pairs
associated with human-readable policy names. For example, user Alice
might have a key pair associated with the names /Alice, /Alice/
Family, and /Alice/Device. Her key is bound to each of these names.
Authentication policies (and trust models) are then expressed as
policy prefix patterns, e.g., /Alice/*. Broadcast messages are
encrypted to policies. For example, Alice might encrypt a message m
to the policy /Bob/*. Only Bob, who owns a private key bound to,
e.g., /Bob/Devices, can decrypt m. (This procedure uses a form of
identity-based encryption called prefix-based encryption. Readers
are referred to [Wu16] for a thorough description.)
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Using prefix- and policy-based encryption, service discovery is
decomposed into two steps: (1) service announcement and (2) key
exchange, similar to [I-D.ietf-dnssd-privacy]. Announcements carry
service identities, ephemeral key shares, and a signature, all
encrypted under the service's desired policy prefix, e.g., /Alice/
Family/*. Upon receipt of an announcement, clients with matching
policy private keys can decrypt the announcement and use the
ephemeral key share to perform an Authenticated Diffie Hellman key
exchange with the service. Upon completion, the derived shared
secret may be used for any further communication, e.g., DNS-SD
queries, if needed.
This system has the following properties:
1. Asymmetric work load: computation for clients is on the order of
advertisements.
2. Mutual identity privacy: Both client and server identities are
hidden from active and passive attackers.
3. Client set size hiding: Policy-based encryption advertisements
hides the number of clients with matching policy keys.
4. Unlinkability: Client initiated connections are unlinkable to
service advertisements (modulo network-layer connection
information, such as advertisement origin and connection
destination).
Author's Address
Christian Huitema
Private Octopus Inc.
Friday Harbor, WA 98250
U.S.A.
Email: huitema@huitema.net
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